ALD/ALE 2022 Session AF4-MoA: Surface Science

Overcoating of metal nanoparticles (NPs) to modulate the performance and thermal stability of catalysts has been proven as an effective method. ^[1] However, most studies focus on the catalytic effects that overcoats bring to the table, while researching their deposition processes is rare, but crucial to tailoring their properties. Here, atomic layer deposition (ALD) of magnesium oxide (MgO) overcoating on platinum (Pt) NPs is developed and investigated. The thickness of the MgO overcoat is precisely controlled by ALD. The prime function of the overcoat is to prevent rapid NP coarsening during the catalyst lifetime, here simulated by high-temperature annealing. During annealing, the NP properties are monitored in real-time by in-situ grazing incidence small angle X-ray scattering (GISAXS)^[2,3].

Mg(EtCp)₂ and H₂O are used as Mg precursor and reactant of the MgO ALD process, respectively, while both 150 ℃ and 200 ℃ deposition temperatures were compared. The ALD process of MgO is initially more selective to Pt than to SiO₂ when using bare SiO₂ and sputtered Pt reference substrates. Via ex-situ XPS after MgO ALD on Pt NP-decorated SiO₂, the evolution of the relative Mg intensity as the function of the number of ALD cycles is shown (Figure 1). It is found that MgO shows a higher growth per cycle at 150 ℃ compared to 200 ℃, consistent with previous work. ^[4] In addition, during the first cycles, MgO mainly covers the Pt NPs, since the Pt signal decreases while the Si contribution remains constant. During the subsequent cycles, the Si intensity also decreases in XPS, suggesting MgO is now grown on the SiO₂.

In-situ GISAXS during annealing (from 25 ℃ to 800 ℃) is adopted to study the influence of MgO overcoating on the Pt NP coarsening behavior. A clear sintering delay is observed for both 150 ℃ and 200 ℃ sample sets with increasing number of MgO ALD cycles. However, the very first cycles (e.g. 1-3) do not contribute too much to the anti-sintering behavior which suggests that decorating the Pt NPs with MgO alone is insufficient for catalyst protection (Figure 2a). The sintering onset temperature is significantly delayed as the number of MgO cycles increases beyond the first cycles, which indicates the NPs thermal stability is mainly enhanced once MgO coats the SiO₂ substrate. This trend is generally applicable for both deposition temperatures, clearly showing a critical amount of MgO is necessary to prevent catalyst coarsening (Figure 2b).

[1] Brandon. J. O’Neill, et al. ACS Catal. 2015, 5, 1804.

[2] Jolien. D, et al. Nat. Comm. 2017, 8, 1074.

[3] Eduardo. S, et al. Nanoscale 2020, 12, 11684.

[4] Burton. B. B, et al. J. Phys. Chem. C 2009, 113, 1939.

To achieve low-impedance tunneling contacts, wet-chemical pretreatments are usually required to prepare the semiconductor surface by removing the native oxide layer. Following this step, it is critical to produce continuous sub-nanometer thin coatings, which are challenging to achieve by atomic layer deposition (ALD) due to surface inhomogeneities and precursor steric interactions that result in island growth during film nucleation. Here, we report a novel atomic layer deposition process that alleviates the need for wet chemical etching and achieves full encapsulation of c-plane gallium nitride (GaN) with an ultimately thin (~3 Å) AlO_x monolayer, which is enabled by the partial transformation of the GaN surface oxide into AlO_x. This is accomplished using repeated cycles of trimethyl aluminum (TMA) and hydrogen (H₂) plasma exposure in a commercial plasma-enhanced ALD reactor (Ultratech Fiji G2). The introduction of ultra-thin AlO_x significantly modifies the physical and chemical properties of the surface, decreasing the work function and introducing new chemical reactivity [1][2]. Electrochemical cyclic voltammetry (CV) measurements show that the ultra-thin film poses a significantly smaller tunneling barrier to charge carrier transport than the thinnest homogenous AlO_x coatings achievable with the conventional TMA/H₂O ALD process.

Depending on the H₂ plasma parameters, the GaO_x surface oxide on GaN can be fully converted into AlO_x, reducing surface band bending and Schottky barrier height at the n-GaN/metal interface. Titanium-contacted n-doped GaN with an ultra-thin interfacial AlO_x layer shows a low contact resistance value and Ohmic behavior even before annealing. Unlike conventional Ohmic contacts to n-type GaN, this annealing-free contact allows for the integration of GaN with other semiconductors such as Si, for which the thermal budget is relatively low (≤ 400 °C). Given the high reactivity of TMA with surface oxides, the presented monolayer AlO_x deposition scheme likely can be extended to other dielectrics and III-V-based semiconductors, with significant relevance to applications in optoelectronics, chemical sensing, and (photo)electrocatalysis.

[1] A. Henning, J. D. Bartl, A. Zeidler, S. Qian, O. Bienek, C.-M. Jiang, C. Paulus, B. Rieger, M. Stutzmann, I. D. Sharp, Adv. Funct. Mater. 2021, 31, (33), 2101441.

[2] J. D. Bartl, C. Thomas, A. Henning, M. F. Ober, G. Savasci, B. Yazdanshenas, P. S. Deimel, E. Magnano, F. Bondino, P. Zeller, L. Gregoratti, M. Amati, C. Paulus, F. Allegretti, A. Cattani-Scholz, J. V. Barth, C. Ochsenfeld, B. Nickel, I. D. Sharp, M. Stutzmann, B. Rieger, J. Am. Chem. Soc. 2021, 143, (46), 19505–19516.

Amorphous titania (am.-TiO₂) has gained broad interest in the field of photocatalysis due to its exceptional disorder-mediated optical and electrical properties compared to crystalline TiO₂ [1–3]. For instance, Ti³⁺ defects within am-TiO₂ can enable essential charge carrier transport through a protective am-TiO₂ photoelectrode coating in photoelectrochemical (PEC) cells [1], and Ti³⁺-mediated visible light active amorphous “black” titania is regarded as a potential material for photocatalytic applications [2]. Atomic layer deposition (ALD) allows for tuning the defect composition and structure of am.-TiO₂ thin films via precursor choices and process parameters. Recent progress in computational analysis of am.-TiO₂ [3] has provided means to accurately correlate experimental insights with theoretical models, which can be utilized to tailor am.-TiO₂ coatings with desired properties.

This work examines how intrinsic titanium and nitrogen defects in am.-TiO₂ can be tailored in a controlled and elegant manner via tuning the ALD growth temperature between 100–200 °C when using tetrakis(dimethylamido)titanium(IV) (TDMAT) and water (H₂O) as the precursors. X-ray photoelectron spectroscopy (XPS) analysis and density functional theory (DFT) calculations allowed us to identify structural disorder-induced penta- and heptacoordinated Ti⁴⁺ ions (Ti_5/7c⁴⁺), which are interrelated to the formation of Ti³⁺ defects in am.-TiO₂ without releasing oxygen, i.e., simultaneous formation of oxygen vacancies and interstitial peroxo species leading to defective but stoichiometric am.-TiO₂. When changing the ALD growth temperature from 100 °C to 200 °C, increase in Ti³⁺ concentration results in “black” TiO₂ and electrical conductivity via polaron hopping mechanism. Furthermore, transient absorption spectroscopy (TAS) shows that the high concentration of Ti³⁺ defects in “black” TiO₂ increases the carrier lifetime to the nanosecond time domain comparable to crystalline low-defect TiO₂. These insights into the formation of Ti³⁺ defects in am.-TiO₂ and into tuning the charge transport properties of ALD grown am.-TiO₂ are beneficial in wide range of applications, such as protective photoelectrode coatings.

[1] P. Nunez, M. H. Richter, B. D. Piercy, C. W. Roske, M. Cabán-Acevedo, M. D. Losego, S. J. Konezny, D. J. Fermin, S. Hu, B. S. Brunschwig, N. S. Lewis, J. Phys. Chem. C 123 (33), 20116–20129 (2019).

[2] V.-A. Glezakou, R. Rousseau, Nat. Mater. 17 (10), 856–857 (2018).

ALD processes are developed and optimized in a limited, 1D process parameter space. The establishment of a steady growth rate within a temperature ‘window’ occurs via a series of experiments, where the independent variable – temperature is varied while holding pulse time constant. Similarly, saturation curves are obtained by varying the independent variable - pulse time (i.e., dose) of the precursors while holding temperature constant. The demonstration of i) a viable temperature window and, ii) saturation curves constitute the establishment of an ALD process. The limitation of these approaches is that process parameter interdependencies cannot be studied. Thus, it is not possible to study the impact of temperature on dose saturation and vice versa. We hypothesize that these interdependencies hold a rich source of undiscovered ALD operation regimes and can lead to efficient process development, robust control and optimization outcomes.

In this talk, we present temperature-time-thickness (TTT) topography maps of ALD processes generated using in situ spectroscopic ellipsometry. Based on the methodology shown by our group recently[1], we demonstrate TTTof several ALD processes including, Al₂O₃, ZnO, CeO₂ and TiO₂ and plasma enhanced ALD (PEALD) of TiO₂. The visualization of these processes in 3D is through a combination of temperature and dose times for thermal ALD processes and as temperature and plasma power for PEALD processes. Saturation regimes of growth rates are observed as 2D surfaces i.e., plateaus and valleys. Precursor adsorption kinetics and thermodynamic parameters are extracted assuming ideal Langmuir adsorption behavior. We propose that a comprehensive database of TTT topographic maps can be used for deeper understanding of processes and to enable robust process control and optimization outcomes.

References:

[1]U. Kumar et al., "In situ ellipsometry aided rapid ALD process development and parameter space visualization of cerium oxide nanofilms," J. Vac. Sci. Technol., A, vol. 39, no. 6, 2021, doi: 10.1116/6.0001329.

By using Atomic Layer Deposition (ALD), amorphous nanotubes can be successfully made of various metal oxides. However, the creation of high quality crystalline nanotubes for example made of sapphire is still a challenging process. To control the crystal structure of ALD based systems, it is indispensable to use microscopy techniques to understand the dynamic processes occurring on atomic scale. In this study, we present a universal approach to create ultrathin crystalline ALD nanotubes by using a comprehensive annealing process of specific core-shell nanowires. In combination with correlative ex situ observations, in situ TEM heating experiments unravel diffusion processes going on at small scales and give insights about temperature induced changes of the metal-ALD interface.
Heating core-shell nanowires (e.g. Cu/ALD-Al₂O₃) at temperatures below 1000°C lead to the creation of hollowed amorphous ALD nanotubes. While the ALD framework is stable, the diffusion of the metallic core material is activated. The conformal ALD shell acts as barrier for the material diffusion and forces the core to continuous diffusion towards the cracked end of the nanowire (Figure 1a). As a first heat-induced phenomenon we observe the creation of voids, which is caused by vacancy agglomeration. Vacancies are induced during the growth process of the metal nanowires at elevated temperatures. However, heating for longer time, the voids and therefore the vacancies get compensated. The thermal activation for the material diffusion can be observed in situ with high resolution imaging (figure 1b). The energy barrier to release an atom from the bonded state can be overcome by heat treatment and the released atoms diffuse within the inner cavity of the ALD tube. The time for releasing atoms scales directly with the temperature (figure 1c). By holding the temperature, no core material is left within the ALD framework and an amorphous nanotube is created. The nanotubes have a high aspect- ratio with lengths up to 40 µm and a wall thickness of 4 nm. At temperatures above 1000°C, a phase transition is induced and a crystalline dense microstructure (κ-Al₂O₃) is achieved (figure 1d).
This type of ultrathin nanotubes shows promising optical and mechanical properties and are the ideal candidate for further functionalization processes.